It is well known that fruits and vegetables, as living organisms, are in a continual state of deterioration from the time of their harvest. Successful marketing of fruits and vegetables depends upon reducing the rate of postharvest quality deterioration by slowing all biological activity. Therefore, attention has been focused on postharvest management of product environment to maintain quality and extend shelf life. The causes of postharvest deterioration and the beneficial effects of postharvest cooling is discussed in detail in "Commercial Cooling of Fruits and Vegetables", Agricultural Publications, University of California, December 1972 and "Methods of Precooling Fruits, Vegetables and Ornamentals", ASHRAE Refrigeration Handbook, American Society of Heating, Refrigeration, and Air-Conditioning Engineers, Chapter 11, 1990. For purposes of this discussion, as a general rule the postharvest precooling of fruits and vegetables is best achieved by rapidly cooling the produce as soon as possible after harvest with cool air having the highest possible humidity. The several methods for produce cooling are discussed below.
Room cooling is a method by which containers holding produce are stored in a room which is brought to a desired temperature and relative humidity. Since air is not forced by or through the produce, cooling is relatively slow, in some cases so slow that the produce may deteriorate to an unacceptable extent in the time required to cool it. Another disadvantage of room cooling is that the warehouse floor space is taken up for a much longer period of time for each load of produce which is cooled. Thus, more rapid cooling methods have been designed.
Vacuum cooling is a method by which certain produce, primarily leafy vegetables, are cooled by enclosing them in air tight chambers and pumping out air and water vapor. However, this method is not well adapted to nonleafy produce and is difficult to control.
Hydrocooling of produce is a method by which the produce is brought in contact with moving cold water. The most common hydrocoolers are conveyor hydrocoolers in which produce in bulk or containers is carrier on a conveyor through a shower of cold water.
Produce may also be cooled by forced-air cooling, also known as pressure cooling. According to this method, air permeable containers of produce are stacked in spaced apart, parallel rows on pallets. Cold air is forced to pass through the openings in the containers and around the produce by creating an air-return plenum between the adjacent rows of containers and utilizing a blower to create a pressure differential across the containers. These systems are described at pages 14-19 of the University of California publication referred to above. Forced-air systems increase the produce cooling rate significantly over the rate offered by room cooling; however, known forced air systems do not provide for independent, sequential loading, cooling and unloading of discrete bay areas to accommodate the arrival and departure of the produce conveyances, for example, tractor trailers. Thus, it becomes an object of the invention to provide operational advantages in the forced-air postharvest cooling of produce.
While most postharvest conditioning of produce involves cooling the produce, as outlined above, in certain circumstances it is desirable to condition the produce with air at an elevated temperature, for example, in the curing of sweet potatoes. As is well known, sweet potatoes continue their metabolic functions after harvest, primarily involving the conversion of certain starches to sugar. It is desirable to cure sweet potatoes after harvest at an elevated temperature on the order of 85.degree. F. for several days to optimize the metabolic functions and to achieve desirable characteristics in the skin of the sweet potatoes. Thus, another object of the invention is to achieve operational advantages in the conditioning of produce at elevated temperatures, for example, in the curing of sweet potatoes.